Miticidal 2-(2-methyl-2,3-dihydro-benzofuran-7-yl)hydrazinecarboxylic acid esters

ABSTRACT

Certain miticidal 2-(2-alkyl-2,3-dihydro-benzofuran-7-yl)-hydrazinecarboxylic acid esters.

DESCRIPTION OF THE INVENTION

It has been found that certain2-(2-methyl-2,3-dihydrobenzofuran-7-yl)hydrazinecarboxylic acid estersare toxic with respect to mites that feed upon plants.

These miticides are described by the generic formula: ##STR1## whereinR³ and R⁴ each is bromine, chlorine, or methyl, m is zero or one, n iszero or one, with the proviso that m+n=1 or 2.

Compounds of Formula I can be prepared by treating the appropriate7-hydrazino-2,3-dihydro-benzofuran with the appropriate methylchloroformate in the presence of a tertiary amine as hydrogen chlorideacceptor. The reaction proceeds according to the equation: ##STR2## Thereaction can be effected by slowly adding one of the reagents to astirred solution of the other reagent in an inert solvent(tetrahydrofuran is a typical example) in the presence of the amine at alow temperature--for example -5° C. to 0° C., then if necessary warmingthe mixture or even heating it at reflux temperature for a timesufficient to ensure completion of the reaction. The formula I speciesis isolated from the reaction mixture, and purified, by conventionalmeans, as is shown in particular instances in the Examples, hereinafter.Methyl chloroformate is a known and readily available material. Asuitable hydrogen chloride acceptor in many instances isN,N-diisopropylethylamine; triethylamine and pyridine are also suitable.

The 7-hydrazino-2,3-dihydro-benzofuran (II) is prepared from thecorresponding 7-nitro-2,3-dihydro-benzofuran according to the reactionsexpressed by the equations ##STR3## wherein A represents ##STR4##

Intermediate IV is prepared by conventional Raney nickel-catalyzedhydrogenation in a Parr shaker of a solution of intermediate III in aninert solvent such as tetrahydrofuran. Intermediate V is prepared bydiazotizing intermediate IV, followed by treatment with sodium sulfite.The diazotization is conventional, effected by treating IV withconcentrated hydrochloric acid at about room temperature, diluting themixture with water, cooling it to about 0° C. and slowly adding anaqueous solution of sodium nitrite to the stirred mixture. Then theresulting diazonium salt solution is added to a cold aqueoussolution/suspension of sodium sulfite, and the resulting mixture isstirred at room temperature to complete the reaction. Intermediate V maybe isolated and further treated, or the crude product may be used toprepare intermediate VI. In either case, aqueous V is treated withsodium dithionite (added in portions to the stirred mixture at roomtemperature), then potassium chloride is added and the mixture isstirred at a moderately elevated temperature (for example, 60°-80° C.)for a time sufficient to complete the reaction. Intermediate VI isrecovered by filtering the mixture. VI then can be converted tointermediate II by mixing it with a lower alkanol, such as methanol,treating the cold (0° C.) mixture with hydrogen chloride, evaporatingthe alkanol, treating the residue with aqueous sodium hydroxide, andextracting the resulting II, using a suitable solvent.

The 7-nitro-2,3-dihydrobenzofuran precursors can be prepared from theappropriate 2-nitrophenol by the general procedures described in U.S.Pat. No. 3,412,110 for the preparation of a similar2,3-dihydrobenzofuran: an alkali metal (M) salt of the phenol (VII) istreated with the appropriate 3-halo-1-alkene, the resulting1-(2-alk-2-enyloxy)-2-nitrobenzene (VIII) is Claisen-rearranged to formthe corresponding 2-(2-alk-2-enyl)-6-nitrophenol (IX), which isring-closed to form the 7-nitro-2,3-dihydrobenzofuran precursor (III).

The reactions proceed according to the equations: ##STR5##

Conversion of VII to VIII can be effected by treating a solution of thephenol (VII), in a solvent such as dimethyl sulfoxide, with an alkalimetal base such as sodium hydroxide or sodium hydride, in the presenceof, or subsequently treating the alkali metal phenoxide thus formed withthe appropriate alkenyl halide, then heating the resulting mixture at amoderately elevated temperature, for example, 80°-120° C.

Claisen-rearrangement of VIII is effected conventionally--convenientlyby heating VIII to a moderately elevated temperature--e.g., 150°-250°C.--in an inert atmosphere.

Ring closure of IX is effected by heating it in the presence of an acid.Suitable acids include the mineral acids, such as hydrochloric,hydrobromic, sulfuric and phosphoric acids. The hydrohalide acids are tobe preferred since they have less tendency to cause side-reactions tooccur. An organic acid such as acetic acid can also be utilized--and maybe used as reaction medium when a mineral acid is used. Ordinarily, theacid is employed as an aqueous solution.

It must be noted that in some cases hydrobromic acid will not besuitable, because at least in part it will add to the double bond,giving the bromoalkyl derivative rather than effecting the cyclization.In such a case, cyclization can be effected by use of a less-reactiveacid, such as hydrochloric acid.

The preparation, isolation and physical properties of typical individualspecies of the compounds of Formula I, in particular instances, aredescribed in the following examples. In each case, the identity of theproduct, and each of any intermediate involved, was confirmed byappropriate chemical and spectral analyses.

EXAMPLE 1 2-(2,3-Dihydro-2,4-dimethylbenzofuran-7-yl)hydrazinecarboxylicAcid Methyl Ester (1)

A solution of 26.1 g of sodium hydroxide in 50 ml of water was addeddrop-by-drop over 5 minutes to a stirred mixture of 100 g of3-methyl-6-nitrophenol, 400 ml of dimethylsulfoxide (DMSO) and 53.6 g of3-chloro-1-propene initially at room temperature. The temperature of themixture rose to 43° C. The mixture was heated at 90°-110° C. for twohours, poured into water and filtered, to give1-(2-propenyloxy)-3-methyl-6-nitrobenzene (1A), as a light yellow solid,m.p.: 30°-32° C.

1A was placed in a flask, which then was purged with nitrogen, and aslow stream of nitrogen was passed through the system while the 1A washeated to 185°-205° C. and stirred at that temperature for 2.5 hours.The mixture was cooled to 25° C., diluted with 400 ml of ether andfiltered through Celite. The solvent was evaporated from the filtrate togive 2-(2-propenyl)-3-methyl-6-nitrophenol (1B), as an amber syrup.

47 g of 1B was added to a mixture of 100 ml of glacial acetic acid and100 ml of 48% aqueous hydrobromic acid and the mixture was heated at114°-115° C. for 2.5 hours. The resulting mixture was stripped todryness, the residue was taken up in 500 ml of ether, and filtered. Thefiltrate was concentrated to 100 ml, 300 ml of hexane was added, themixture was chilled and filtered to give2,3-dihydro-2,4-dimethyl-7-nitrobenzofuran (1C), as a tan solid, m.p.:69°-71° C.

A mixture of 24.3 g of 1C, 200 ml of tetrahydrofuran (THF) and 2 g ofactivated Raney nickel catalyst was treated with hydrogen (40 psig) in aParr shaker three hours, then filtered. The solvent was evaporated fromthe filtrate to give 7-amino-2,3-dihydro-2,4-dimethylbenzofuran (1D), asan amber syrup.

A mixture of 50 ml of concentrated hydrochloric acid and 19.8 g of 1Dwas stirred, warmed to 80° C. and allowed to stand at room temperaturefor 18 hours. Then 50 ml of ice water was added, the mixture was cooledto 0° C. and stirred while a solution of 9.6 g of sodium nitrite in 20ml of water was added drop-by-drop over 15 minutes to the cold stirredmixture. The cold mixture was stirred for an additional hour and thenwas added to a cold stirred mixture of 100 g of sodium sulfite in 180 mlof water. The resulting mixture was stirred at room temperature for sixhours, then 21.9 g of sodium dithionite was added, in portions. Theresulting mixture was stirred at room temperature for 18 hours, then at75° C. while 150 g of potassium chloride was added. The resultingmixture was stirred at 75° C. for 30 minutes, then at room temperaturefor two hours, then was cooled to 5° C. and filtered. The filter cakewas air dried, mixed with 400 ml of methanol, the mixture was chiled to0° C. and an excess of anhydrous hydrogen chloride was added. Themethanol was evaporated under reduced pressure, the residue taken up inwater, 50% aqueous sodium hydroxide solution was added, and theresulting mixture was extracted with ether. The extract was dried(MgSO₄), filtered and the solvent was evaporated to give2,3-dihydro-2,4-dimethyl-7-hydrazinobenzofuran (1E), as an amber syrup.

15.6 g of 1E was mixed with 150 ml of THF and 11.3 g ofN,N-diisopropylethylamine. The mixture was cooled to -5° C. and 9.1 g ofmethyl chloroformate was added drop-by-drop to the stirred mixture,which then was stirred at 0° C. for two hours. The mixture then waspoured over ice water and extracted with ether. The extract was dried(MgSO₄), filtered and the solvent was evaporated. The residue was columnchromatographed on silica gel, using a 1:4:20 v:v:v mixture oftetrahydrofuran, ethyl acetate and hexane as eluent, to give 1, as a tansolid, m.p.: 90°-92° C.

EXAMPLE 2

2-(2,3-Dihydro-2,5-dimethylbenzofuran-7-yl)hydrazinecarboxylic acidmethyl ester (2) was prepared as a tan solid, m.p.: 92°-94° C., from4-methyl-2-nitrophenol, and 3-chloropropene according to the proceduresdescribed in Example 1.

EXAMPLE 32-(4-Chloro-2,3-dihydro-2,5-dimethylbenzofuran-7-yl)hydrazinecarboxylicAcid Methyl Ester (3)

283.0 g of 3-chloro-4-methylaniline, 1000 ml of water and 500 ml ofconcentrated sulfuric acid were mixed, initially at room temperature.The temperature of the mixture rose to 95° C. The mixture was stirred atroom temperature for 30 minutes, cooled to 5° C. and a solution of 145 gof sodium nitrite in 500 ml of water was slowly added (1.5 hours) to thestirred mixture at 5°-10° C., then the mixture was stirred at about 5°C. for one hour. The resulting solution was added over two hours to astirred solution of 1.2 liters of concentrated sulfuric acid in 2 litersof water, at 105°-110° C., and the resulting mixture was stirred at 110°C. for one hour, then held at room temperature for 18 hours. Then 3.5liters of hexane was added, the mixture was stirred for 30 minutes,allowed to stand, and the two liquid phases were separated. The hexanephase was filtered, washed with water, dried (MgSO₄) and concentrated todryness to give crude (91%) 3-chloro-4-methylphenol (3A).

245.0 g of 3A was mixed with 1.6 liters of glacial acetic acid, and theresulting solution was stirred at 8°-10° C. while 109.6 g of 90% aqueousnitric acid was added (two hours). The mixture was stirred at 10°-15° C.for five hours, poured over ice water and extracted with ether/hexane.The extract was washed with water, dried and stripped to dryness. Theresidue was stirred with 3 liters of hexane and the hexane solution wasfiltered through silica gel. The filtrate was held at -20° C. for 16hours, filtered, and the filtrate was concentrated to one liter,charcoaled, then held at -30° to -35° C. for two hours and filtered. Thesolid products were combined to give 3-chloro-4-methyl-6-nitrophenol(3B).

A mixture of 93.4 g of 3B, 750 ml of DMSO, 48 g of 50% aqueous sodiumhydroxide and 45.9 g of 3-chloro-1-propene was stirred for 8 hours at75°-80° C., then for 16 hours at room temperature, was mixed with 3liters of ice water and the resulting mixture was extracted withmethylene chloride. The extract was washed with water, dried (MgSO₄) andevaporated to dryness. The residue was placed in a flask purged withnitrogen, in a nitrogen atmosphere was heated slowly to 190° C. (onehour) and held at 185°-195° C. for two hours, cooled to roomtemperature, and dissolved into 250 ml of acetic acid. To this solution150 ml of 48% aqueous hydrogen bromide solution was added, the mixturewas stirred at reflux for three hours, mixed with 2 liters of ice waterand the resulting mixture was extracted with a 1:1 v:v mixture of hexaneand ether. The extract was washed with water, 5% aqueous sodiumbicarbonate solution, dried (MgSO₄), charcoaled and concentrated todryness. The residue was crystallized from 500 ml of 9:1 v:vhexane/ether mixture, giving2,3-dihydro-2,5-dimethyl-4-chloro-7-nitrobenzofuran (3C), m.p.:114°-116° C.

51.0 g of 3C was dissolved in 600 ml of THF. The solution was divided inhalf, and each half was hydrogenated (40-50 psig hydrogen pressure) inthe presence of 1 g of palladium-on-charcoal catalyst. The finalcombined reaction mixtures were filtered, the filtrates were combined,dried (MgSO₄) and evaporated to dryness, to give7-amino-2,3-dihydro-2,5-dimethyl-4-chlorobenzofuran (3D).

19.75 g of 3D, 200 ml of water and 100 ml of concentrated hydrochloricacid were mixed, cooled to 0° C. and a solution of 7.3 g of sodiumnitrite in 50 ml of water was slowly added (one hour) to the stirredmixture at 0°-5° C. The mixture was stirred at 25° C. for one hour, then55.2 g of potassium carbonate was slowly added (one hour) at about 5° C.The resulting mixture was cooled to 0° C. and slowly added (over 30minutes) to a solution of 50.4 g of sodium sulfite in 300 ml of water at5°-10° C. The mixture was stirred at 5°-10° C. for three hours, and atroom temperature for 16 hours. Then 20.9 g of sodium dithionite wasadded, the mixture was stirred at room temperature for two hours, then80° C. for 15 minutes. Then 200 g of potassium chloride was added, themixture was cooled and stirred at 5° C. for two hours, filtered and thecollected solid was air-dried. The solid then was mixed with 250 ml ofmethanol, and with stirring and cooling (5°-15° C.), 5 g of anhydroushydrogen chloride was bubbled into the mixture, which then was stirredat room temperature for three hours. The methanol was evaporated underreduced pressure, at 30°-40° C. The residue was added to 500 ml of icewater and 10 g of 50% aqueous sodium hydroxide solution. The resultingmixture was extracted with ether, the extract was dried (MgSO₄) andevaporated to dryness. The residue was dissolved in 200 ml oftetrahydrofuran and 12.9 g of N,N-diisopropylethylamine was added. Themixture was stirred at 5° C. while 5.7 g of methyl chloroformate wasadded over 15 minutes at 5°-10° C. The mixture was poured into ice waterand extracted with ether. The extract was dried (MgSO₄) and evaporatedto dryness. The residue was washed with 1:1 v:v ether/hexane mixture anddried, to give 3, as a white solid, m.p.: 157°-160° C.

EXAMPLE 42-(2,3-Dihydro-5-chloro-2,4-dimethylbenzofuran-7-yl)hydrazinecarboxylicAcid Methyl Ester (4)

4 was prepared, as tan solid, m.p.: 152°-154° C., from4-chloro-6-nitro-m-cresol and 3-bromo-1-propene by the proceduresdescribed in Example 3 for the preparation of 1 from3-methyl-6-nitrophenol.

Compounds of Formula I have been found to be toxic to mites that feed onplants, with little or no toxicity to other pests which feed on plants.Compounds of Formula I have been found to be effective with respect toresistant mites as well as to susceptible mites.

Accordingly, the invention includes a method for combattingplant-feeding mites which comprises applying to the foliage of theplants to be protected an effective amount of a compound of Formula I.

for application, the compound of the invention ordinarily is appliedmost effectively by formulating it with a suitable inert carrier orsurface-active agent, or both. The invention, therefore, also includescompositions suitable for combatting mites, such compositions comprisingan inert carrier or surface-active agent, or both, and as activeingredient at least one compound of the invention.

The term "carrier" as used herein means an inert solid or liquidmaterial, which may be inorganic or organic and of synthetic or naturalorigin, with which the active compound is mixed or formulated tofacilitate its application to the plant, seed, soil or other object tobe treated, or its storage, transport and/or handling. Any of thematerials customarily employed in formulating pesticides are suitable.

Suitable solid carriers are natural and synthetic clays and silicates,for example, natural silicas such as diatomaceous earths; magnesiumsilicates, for example, talcs; magnesium aluminum silicates, forexample, attapulgites and vermiculites; aluminum silicates, for example,kaolinites, montmorillonites and micas; calcium carbonate; calciumsulfate; synthetic hydrated silicon oxides and synthetic calcium oraluminum silicates; elements such as, for example, carbon and sulfur;natural and synthetic resins such as, for example, coumarone resins,polyvinyl chloride and styrene polymers and copolymers; bitumen; waxessuch as, for example, beeswax, paraffin wax, and chlorinated mineralwaxes; solid fertilizers, for example, superphosphates; and ground,naturally-occurring, fibrous materials, such as ground corncobs.

Examples of suitable liquid carriers are water, alcohols such asisopropyl alcohol and glycols; ketones such as acetone, methyl ethylketone, methyl isobutyl ketone and cyclohexanone; ethers such ascellosolves; aromatic hydrocarbons such as benzene, toluene and xylene;petroleum fractions such as kerosene, light mineral oils; chlorinatedhydrocarbons such as carbon tetrachloride, perchloroethylene andtrichloromethane. Also suitable are liquefied, normally vaporous andgaseous compounds. Mixtures of different liquids are often suitable.

The surface-active agent may be an emulsifying agent or a dispersingagent or a wetting agent; it may be nonionic or ionic. Any of thesurface-active agents usually applied in formulating herbicides orinsecticides may be used. Examples of suitable surface-active agents arethe sodium and calcium salts of polyacrylic acids and lignin sulfonicacids; the condensation products of fatty acids or aliphatic amines oramides containing at least 12 carbon atoms in the molecule with ethyleneoxide and/or propylene oxide; fatty acid esters of glycerol, sorbitan,sucrose or pentaerythritol; condensates of these with ethylene oxideand/or propylene oxide; condensation products of fatty alcohols or alkylphenols, for example, p-octylphenol or p-octylcresol, with ethyleneoxide and/or propylene oxide; sulfates or sulfonates of thesecondensation products, alkali or alkaline earth metal salts, preferablysodium salts, of sulfuric or sulfonic acid esters containing at least 10carbon atoms in the molecule, for example, sodium lauryl sulfate, sodiumsecondary alkyl sulfates, sodium salts of sulfonated castor oil, andsodium alkylaryl sulfonates such as sodium dodecylbenzene sulfonate; andpolymers of ethylene oxide and copolymers of ethylene oxide andpropylene oxides.

The compositions of the invention may be prepared as wettable powders,dusts, granules, solutions, emulsifiable concentrates, emulsions,suspension concentrates and aerosols. Wettable powders are usuallycompounded to contain 25-75% by weight of active compound and usuallycontain, in addition to the solid carrier, 3-10% by weight of adispersing agent, 2-15% of a surface-active agent and, where necessary,0-10% by weight of stabilizer(s) and/or other additives such aspenetrants or stickers. Dusts are usually formulated as a dustconcentrate having a similar composition to that of a wettable powderbut without a dispersant or surface-active agent, and are diluted in thefield with further solid carrier to give a composition usuallycontaining 0.5-10% by weight of the active compound. Granules areusually prepared to have a size between 10 and 100 BS mesh (1.676-0.152mm), and may be manufactured by agglomeration or impregnationtechniques. Generally, granules will contain 0.5-25% by weight of theactive compound, 0-1% by weight of additives such as stabilizers, slowrelease modifiers and binding agents. Emulsifiable concentrates usuallycontain, in addition to the solvent and, when necessary, cosolvent,10-50% weight per volume of the active compound, 2-20% weight per volumeemulsifiers and 0-20% weight per volume of appropriate additives such asstabilizers, penetrants and corrosion inhibitors. Suspensionconcentrates are compounded so as to obtain a stable, non-sedimenting,flowable product and usually contain 10-75% weight of the activecompound, 0.5-5% weight of dispersing agents, 1-5% of surface-activeagent, 0.1-10% weight of suspending agents, such as defoamers, corrosioninhibitors, stabilizers, penetrants and stickers, and as carrier, wateror an organic liquid in which the active compound is substantiallyinsoluble; certain organic solids or inorganic salts may be dissolved inthe carrier to assist in preventing sedimentation or as antifreezeagents for water.

Of particular interest in current practice are the water-dispersiblegranular formulations. These are in the form of dry, hard granules thatare essentially dust-free, and are resistant to attrition on handling,thus minimizing the formation of dust. On contact with water, thegranules readily disintegrate to form stable suspensions of theparticles of active material. Such formulations contain 90% or more byweight of finely divided active material, 3-7% by weight of a blend ofsurfactants, which act as wetting, dispersing, suspending and bindingagents, and 1-3% by weight of a finely divided carrier, which acts as aresuspending agent.

Aqueous dispersions and emulsions, for example, compositions obtained bydiluting a wettable powder or a concentrate according to the inventionwith water, also lie within the scope of the present invention. The saidemulsions may be of the water-in-oil or of the oil-in-water type, andmay have thick, mayonnaise-like consistency.

It is evident from the foregoing that this invention contemplatescompositions containing as little as about 0.0001% by weight to as muchas about 95% by weight of a compound of the invention as the activeingredient.

The compositions of the invention may also contain other ingredients,for example, other compounds possessing pesticidal, especiallyinsecticidal, acaricidal or fungicidal properties, as are appropriate tothe intended purpose.

The method of applying a compound of the invention to control mitescomprises applying the compound, ordinarily in a composition of one ofthe aforementioned types, to a locus or area to be protected from themites, such as the foliage and/or the fruit of plants. The compound, ofcourse, is applied in an amount sufficient to effect the desired action.This dosage is dependent upon many factors, including the carrieremployed, the method and conditions of the application, whether theformulation is present at the locus in the form of an aerosol, or as afilm, or as discrete particles, the thickness of film or size ofparticles, and the like. Proper consideration and resolution of thesefactors to provide the necessary dosage of the active compound at thelocus to be protected are within the skill of those versed in the art.In general, however, the effective dosage of the compound of theinvention at the locus to be protected--i.e., the dosage which the mitecontacts--is of the order of 0.001 to 0.5% based on the total weight ofthe formulation, though under some circumstances the effectiveconcentration will be as little as 0.0001% or as much as 2%, on the samebasis.

It has been found that the stability of compounds of this invention maybe adversely affected by some surfactants, resulting in decomposition ofsuch compounds during storage of the formulation containing thesurfactant. Also, it appears that the stability of compounds of theinvention may be adversely affected by mineral carriers. Thischaracteristic of the compounds of the invention must be taken intoaccount when formulating them.

MITICIDAL ACTIVITY

Toxicity of compounds of this invention with respect to mites weredetermined as follows:

Adult female two-spotted spider mites (Tetranychus urticae (Koch) weretested by placing 50-75 mites on the bottom side of leaves of pinto beanplants. The leaves were sprayed with dilutions of acetone solution oftest compound into water containing an emulsifier and kept underlaboratory conditions for about 20 hours at which time mortality countswere made. The tests were conducted employing several different dosagerates of test compounds.

In each set of tests, identical tests were conducted using Parathion asa standard for comparison.

In each instance, the toxicity of the test compound was compared to thatof the standard pesticide (Parathion), its relative toxicity then beingexpressed in terms of the relationship between the amount of the testcompound and the amount of the standard pesticide required to producethe same percentage (50%) of mortality in the mites. By assigning thestandard pesticide an arbitrary rating of 100, the toxicities of thetest compound was expressed in terms of the Toxicity Index, whichcompares the toxicity of the test compound of the invention with that ofthe standard pesticide. That is to say, a test compound having aToxicity Index of 50 would be half as active, while one having aToxicity Index of 200 would be twice as active, as the standardpesticide. The results are set forth in Table I.

                  TABLE I                                                         ______________________________________                                        Compound    Toxicity Index, Two-Spotted                                       Number      Spider Mites                                                      ______________________________________                                        1           290                                                               2           290                                                               3           120                                                               4           390                                                               ______________________________________                                    

In similar standardized tests, compounds of Formula I were found to havelittle or no toxicity with respect to houseflies, pea aphids, and cornearworms.

In a similar standardized tests, Compound 1 was found to be about astoxic with respect to a strain of two-spotted spider mites that areresistant to organophosphorus insecticides as with respect to a strainof two-spotted spider mites that are not so resistant.

In addition to their utility as miticides, compounds of Formula I alsohave utility as precursors for the miticidal and insecticidal compoundsof copending application Ser. No. 643,329.

We claim:
 1. A compound of the formula ##STR6## wherein m and n each isone, one of R³ and R⁴ is methyl and the other is chlorine.
 2. A compoundaccording to claim 1 wherein R³ is methyl and R⁴ is chlorine.
 3. Acompound according to claim 1 wherein R³ is chlorine and R⁴ is methyl.4. A method for protecting a plant from mites that comprises applying tothe plant a miticidally effective dosage of a compound of the formula:##STR7## wherein R³ and R⁴ each is bromine, chlorine or methyl, m iszero or one, n is zero or one, with the proviso that m+n=one or two. 5.A method according to claim 4 wherein the compound is one in which R³ ismethyl and R⁴ is chlorine.